1. Field of the Invention
This invention relates to an ultrasonic transducer and, more particularly, to an ultrasonic transducer well-suited for use underwater or for diagnosis of a living body.
2. Description of the Prior Art
Ultrasonic transducers generally make use of a piezoelectric member which, as is well-known in the art, may be a ceramic piezoelectric member consisting of lead zirconate titanate, barium titanate, lead titanate or the like, a piezoelectric polymer member consisting of polyvinylidene fluoride or the like, or a composite piezoelectric member consisting of a polymer and a ceramic. The piezoelectric polymer member and the composite piezoelectric member have much higher workability and a lower acoustic impedence than the ceramic piezoelectric member and for this reason have come to be widely employed in ultrasonic transducers for use underwater or for diagnosis of a living body.
FIG. 2 illustrates an example of the cross-sectional structure of an ultrasonic transducer using a piezoelectric polymer member. The transducer includes a piezoelectric body 1 obtained by providing each of the opposing main surfaces of a flat, plate-shaped piezoelectric polymer member with an electrode, not shown. The piezoelectric body 1 has a reflector 2 deposited on one of its surfaces and is affixed to a support member 3 via the reflector 2. Numeral 5 denotes a specimen undergoing examination.
It is known that when an ultrasonic transducer of this construction is driven by a .lambda..sub.1 /4 driving waveform, namely when a .lambda..sub.1 /4 drive method is employed, sensitivity is maximized if the thickness of the piezoelectric body 1 is made .lambda..sub.1 /4. Note that .lambda..sub.1 represents the sonic wavelength inside the piezoelectric body 1 at a frequency which is one half the free resonance frequency of the piezoelectric body 1.
It is known that when a single pulse voltage is applied to an ultrasonic transducer, an ultrasonic wave is radiated toward the reflector. As used herein, the term "sensitivity" S is the maximum value V.sub.P of signal voltage generated in the transducer, and the term "response" R is the dispersion of the signal waveform along a time axis about the maximum value V.sub.P. Thus, the response R is in units of time and may correspond, for example, to a time interval during which the signal waveform is within one-tenth or one-hundredth of its maximum value V.sub.P.
Ordinarily, the thickness of the reflector 2 is set to .lambda..sub.2 /4 (where .lambda..sub.2 is the sonic wavelength inside the reflector). However, it has been proposed in the specification of Japanese Patent Publication for Opposition No. 59-9000 to reduce the thickness of the reflector to below .lambda..sub.2 /4 in order to exploit the flexibility possessed by the piezoelectric polymer member and attain a higher efficiency and a wider band.
The graph of FIG. 3 shows the results of analyzing the sensitivity and response of an ultrasonic transducer when the thickness of the reflector 2 is varied from 0 to .lambda..sub.2 /3. The analytic method is in line with the principle of analyzing, by a gradualistic method (or sequence definition equation method), the amplitude of a pressure wave produced under the application of a single voltage pulse, as set forth in the specification of Japanese Patent Application Laid-Open No. 60-185499. The thickness of the reflector 2 is plotted along the horizontal axis, and both the sensitivity (relative values) and response of the ultrasonic transducer are plotted along the vertical axis. The characters S and R indicate the analytical data representative of sensitivity and response, respectively, and f stands for frequency (MHz). The various materials analyzed and the corresponding acoustic impedences are shown in Table 1. It will be understood from FIG. 3 that, for a reflector 2 having a thickness within the range .lambda..sub.2 /30 (=4.lambda..sub.2 /120) to 11.lambda..sub.2 /60 (=22.lambda..sub.2 /120) (A--A' line in FIG. 3), sensitivity and response are higher than for a reflector having a thickness of .lambda..sub.2 /4 (=30.lambda..sub.2 /120). This agrees with the subject matter disclosed in the aforementioned specification of Japanese Patent Publication No. 59-9000.
With an ultrasonic transducer formed from a polymeric or composite piezoelectric member, the surface of the piezoelectric body 1, namely the surface that receives the ultrasonic wave, has an acoustic matching layer 4 deposited thereon to serve as a protective layer for protecting the piezoelectric member and the electrode formed on the abovementioned surface. As shown in FIG. 1, the ultrasonic transducer ordinarily is used by being brought into contact with the specimen 5 through the intermediary of the acoustic matching layer 4.
However, the ultrasonic transducer described in the abovementioned specification does not possess an acoustic matching layer and the specification does not go beyond a description of the polymeric piezoelectric body. In general, the thickness and acoustic impedence of the acoustic matching layer are important factors which influence various characteristics of the ultrasonic transducer. Common technical knowledge in the prior art is that .lambda..sub.3 /4 is the preferred acoustic matching layer thickness (where .lambda..sub.3 /4 is the sonic wavelength inside the acoustic matching layer). However, whether .lambda..sub.3 /4 is indeed the optimum thickness of the acoustic matching layer has not been investigated, and neither has the relationship between the acoustic matching layer thickness and the reflector thickness.